1. Field of the Invention
The present invention relates to a transfer robot used for semiconductor manufacturing equipment, liquid crystal display processing equipment and the like. More particularly, the present invention relates to a transfer robot for transferring workpieces between processing chambers under a vacuum.
2. Description of the Related Art
Conventionally, use has been made of various kinds of transfer robots designed for semiconductor manufacturing equipment, liquid crystal display processing equipment and the like. FIGS. 15-17 of the accompanying drawings illustrate an example of conventional transfer robot.
As shown in FIG. 17, the conventional transfer robot is provided with a handling member 84. Though not illustrated, an object to be processed (called "workpiece" hereinafter), such as a silicon wafer, is placed on the handling member 84. The handling member 84, which is carried by an arm mechanism, is arranged to move horizontally in a straight line as well as to rotate in a horizontal plane around a central axis P1. A plurality of processing chambers 71-76 for performing predetermined processing are disposed around the central axis P1. With the use of the transfer robot, the workpiece is automatically brought to and taken away from a selected one of the processing chambers 71-76.
Referring to FIG. 15, the conventional transfer robot includes a rotatable base 81 and a first arm 82. The rotatable base 81 is caused to rotate about a first axis P1 by a driving motor, while the first arm 82 is caused to rotate about the first axis P1 by another driving motor which is fixed to the rotatable base 81.
In FIG. 15, reference number 83 refers to a second arm which is rotatable about a second axis Q1 relative to the first arm 82, while reference numeral 84 refers to a handling member which is rotatable about a third axis R1 relative to the second arm 83.
Reference numeral 85 refers to a first rotation-transmitting member which is fixed to the rotatable base 81 coaxially with the first axis P1, while reference numeral 86 refers to a second rotation-transmitting member which is fixed to the second arm 83 coaxially with the second axis Q1. Reference numeral 87 refers to a third rotation-transmitting member fixed to the first arm 82 coaxially with the second axis Q1, while reference numeral 88 refers to a fourth rotation-transmitting member fixed to the handling member 84 coaxially with the third axis R1.
A first connecting member 89 is provided between the first rotation-transmitting member 85 and the second rotation-transmitting member 86. Also, a second connecting member 90 is provided between the third rotation-transmitting member 87 and the fourth rotation-transmitting member 88. The distance S between the first and second axes P1, Q1 is equal to the distance between the second and third axes Q1, R1. The radius ratio of the first rotation-transmitting member 85 to the second rotation-transmitting member 86 is 2 to 1. The radius ratio of the fourth rotation-transmitting member 88 to the third rotation-transmitting member 87 is also 2 to 1.
Chain sprockets or pulleys may be used for the first to fourth rotation-transmitting members 85-88. Correspondingly, the first and second connecting members 89, 90 may be chains or timing belts.
Reference will now be made to the operation of the arm mechanism of the conventional transfer robot.
At the outset, it is assumed that the rotatable base 81 is kept stationary, and that the first, second and third axes P1, Q1, R1 are initially located in a common straight line, as shown in FIG. 16. Starting from this state, the first arm 82 is rotated counterclockwise through an angle .theta. about the first axis P1.
During the above operation, the first rotation-transmitting member 85 is fixed in position, while the second axis Q1 is moved counterclockwise around the first axis P1 through the angle .theta.. (Thus, the second axis Q1 is shifted from the initial position to a new position Q11.) As a result, a Y1-side portion of the first connecting member 89 is wound around the first rotation-transmitting member 85, whereas a Y2-side portion of the same connecting member is unwound from the first rotation-transmitting member 85.
Thus, as shown in FIG. 16, the first connecting member 89 is moved in a direction indicated by arrows a1 and a2. As a result, the second rotation-transmitting member 86 is rotated clockwise about the second axis Q1.
As mentioned above, the radius ratio of the first rotation-transmitting member 85 to the second rotation-transmitting member 86 is 2 to 1. Thus, when the first arm 82 is rotated counterclockwise about the first axis P1 through the angle .theta., the second rotation-transmitting member 86 is rotated clockwise about the second axis Q11 through an angle 2.theta..
At this time, since the second rotation-transmitting member 86 is fixed to the second arm 83, the second rotation-transmitting member 86 and the second arm 83 are rotated clockwise about the second axis Q1 through an angle 2.theta..
If the second arm 83 did not change its orientation relative to the first arm 82, the third axis R1 would be brought to an R11 position shown by broken lines. Actually, however, the second rotation-transmitting member 86 is rotated clockwise about the second axis Q11 through an angle 2 .theta.. Therefore, the third axis R11 is moved clockwise about the second axis Q11 through the same angle 2 .theta. to be brought to the R12 position. This means that the third axis R12 remains in the straight line extending through the first and the third axes Pl and R1 even while the first arm 82 is being rotated counterclockwise about the first axis P1 through an angle .theta..
When the second arm 83 is rotated clockwise about the second axis Q11 through an angle 2 .theta., thereby bringing the third axis R11 to the R12 position, a Y2-side portion of the second connecting member 90 is wound around the third rotation-transmitting member 87, whereas a Y1-side portion of the same connecting member is unwound from the third rotation-transmitting member 87.
As a result, the second connecting member 90 will be shifted in a direction b1-b2 shown in FIG. 16. Thus, the fourth rotation-transmitting member 88 is rotated counterclockwise about the third axis R12.
When the second arm 83 is rotated clockwise about the second axis Q11 through an angle 2 .theta. as stated above, the fourth rotation-transmitting member 88 is rotated counterclockwise about the third axis R12 through an angle .theta. (since the radius ratio of the fourth rotation-transmitting member 88 to the third rotation-transmitting member 87 is 2 to 1). As a result, a point C0 of the fourth rotation-transmitting member 88 is brought to a position C1 on the straight line passing through the first and the third axes P1, R12.
Upon rotation of the first arm 82 about the first axis P1 in the counterclockwise direction as described above, the handling member 84 is moved along the line passing through the first and the third axes P1, R1. During this operation, the handling member 84 does not changed its attitude or orientation since it is fixed to the fourth rotation-transmitting member 88.
The transfer robot having the above-described arrangement is installed at the center of the processing chambers 71-76, as shown in FIG. 17. Workpieces are transferred by the transfer robot between these chambers 71-76.
Though useful in many respects, the conventional transfer robot has been found disadvantageous in the following points.
First, as shown in FIG. 15, the second arm 83 incorporates the fourth rotation-transmitting member 88 and the second connecting member 90. In this arrangement, the second arm 83 is rendered to have an unduly great thickness H1.
Second, since the non-illustrated driving motor for actuating the arm mechanism is mounted on the rotatable base 81, the motor is rotated together with the base 81 around the axis P1. In the conventional transfer robot, use is made of a power supply cable for connecting the driving motor to an external power source. Thus, when the driving motor is moved around the central axis P1, the power supply cable may be wounded about a shaft. Clearly, when the cable has been wounded on the shaft too many times and yet the driving motor continues to be moved around the central axis PI, the cable may be damaged (snapped at worst).
In order to avoid such a problem, the rotation of the base 81 should be stopped before the rotation angle of the base 81 goes beyond a predetermined limit (540.degree. forexample). However, for controlling the rotation of the rotatable base 81, additional devices such as a monitor and a rotation controlling unit may be needed. Disadvantageously, such additional devices will render the transfer robot unduly expensive. Besides, the restriction of the rotation angle of the base 81 tends to make the conventional transfer robot less usable.